Optical fiber amplifiers are widely used for signal amplification in optical data transmitting networks based on wavelength-division multiplexing (WDM). Changes in network configuration, component failures, fiber breaks or protection switching can cause abrupt changes of optical input power. These changes cause fast changes of amplifier output power which can be transferred to other wavelengths due to nonlinear fiber effects and the non-ideal dynamic properties of erbium-doped fiber amplifiers (EDFAs). These changes can propagate to other sites leading to optical power fluctuations across the whole network and possibly to oscillations. Thus, even channels that are not directly affected by the switching operations or failures can suffer from some performance degradation at the receivers.
Furthermore, gain variations can also accumulate in a cascade of amplifiers. Thus, even small gain variations can result in significant power changes at the receivers. Therefore, efficient amplifier control techniques are required that allow to keep the inversion and as a consequence the gain profile of an amplifier or an amplifier stage relatively constant even if the input power changes.
Fast electronic control architectures are currently the most economical solution to stabilize the gain of EDFAs. Commonly, feedback architectures are used since they allow to adjust the gain or output power to given target values and to compensate for control errors. However, purely feedback based controllers cannot meet the transient performance requirements for dynamically reconfigured networks. Fortunately, feedback controllers can be complemented by a feedforward controller. The combination of the two types of controllers provides quick response to any changes with the feedback system cleaning up for any error in the predetermined adjustment made by the feedforward control.
On the other hand, cost reduction has become a continuing task. Therefore pump power splitting has become a widely used technique to reduce amplifier cost. If pump power splitting is applied to amplifier stages that are separated by a component afflicted with delay such as a dispersion compensating fiber (DCF) unacceptable poor transient performance is gained. Therefore, pump splitting is typically applied only to stages that are all before the DCF or all after the DCF.